1970 Velocette Thruxton recommissioning

 Yesterday found me resetting and checking cam timing the Thruxton engine. I began this the day before but found that there was not enough clearance to set the specified gaps for both the intake and exhaust as specified in my manuals. [Haynes Workshop manual-BMS Service Series book- and Reliable Service Manual ]

Since the engine was partially disassembled before it arrived here in the shop; the previous owner had removed the distributor, rockers and pushrods. I couldn't be certain as to whether parts in boxes were from this engine or spares. Once the rockers and pushrods were in place it was evident that you could not set the needed 0.053" gap on the inlet and 0.052" for the exhaust for checking cam timing.  The foot of the adjuster side of the rocker was fouling the pushrods.

[NOTE to keep in mind:  on this Thruxton the cam is a M17/8 and uses the 0.053" intake and 0.052" exhaust for gaps when timing the cam.  The M17/7 which is found on Vipers and MSS models and possibly others will want the gap set at 0.030"  for both the intake and exhaust.  As you read on further, all my comments will relate to the M17/8 cam with quieting ramps. Hence taking a few times to measure it again and again. The most important of all the measurements will be the opening of the intake]

Observations? The pushrods might not be from this engine, the cam followers might not be from this engine, the rocker foot that presses on the valve stem looked to be almost unworn. Maybe the rockers had been replaced? And from there, maybe the valves had been replaced; they looked good and not recessed in the heads. Perhaps the valve stems are taller than what is standard? Knowing that people make their own pushrods for Thruxton engines I thought about shortening them.  Before anything though, I gave Ed G. a call incase I was overlooking something. 

Sadly, Ed wasn't feeling well, but we did come to few thoughts; the first was whether the the engine height had changed. Since I was reusing the two compression plates and replaced the two paper gaskets, I was confident that had not changed. The head gasket was replaced but was not thinner than the head gasket that came out. Ed suggested to use two head gaskets for setting the cam timing, then remove the added gasket after setting the cam; but noted this a pain as you have to assemble/disassemble more times.  The thought to shorten the pushrods was passed on; not quite as a straight forward process to do.  Removing some material from the rocker where it was fouling the pushrod seemed to be a logical solution and would not affect the overall mechanics.

cam follower feet examples

cam follower feet examples

cam follower feet examples

cam follower feet examples

cam follower feet examples

At this point we go back to the old hand built brit bike mantra; Put it together-take it apart-put it together-take it apart-put it together-take it apart-put it together-take it apart-put it together-take it apart. After several passes between small grinding sessions and reinstalling the rocker box, we had success and were able to set the rockers to the 0.053"intake and 0.052exhaust! On to timing the cam.

The last time I did the cam timing was on the MSS engine. At first, it all reads to be confusing; use a timing disk, set TDC by the positive stop method, removed the exhaust pushrod, set the intake at 0.053" then read degrees at which point the intake closes and opens. Now do the same for the exhaust but set to 0.052". However, if there are no marks on the timing gears where to start?  Again, I'm thankful that Ed had an answer for that too!

Setting and checking the gap on the exhaust for cam timing

 What Ed had told me was simple, elegant and and easy to remember.  The most important to set is the intake opening, since the cam is one piece both intake and exhaust lobes are 'timed' to each other. Set the 0.053" gap on the intake, remove the intermediate gear and the exhaust pushrod. Set the crankshaft position to the indicated degree you want the intake to begin opening.  In this case, I want the intake to open begin opening at 45` BTDC [M17/8 cam timing;  45`/55`/65`/35`]. With the gap set and the crankshaft set, rotate the cam  clockwise until the foot is pressing on the intake rocker foot and no up and down play can be felt in push rod tube. Hold this firmly and without moving the crankshaft, install the intermediate gear back in, you might need to SLIGHTLY rotate the crankshaft to get it to mesh. Now double check everything to triple check; I tend forget which direction the crankshaft/gears/ magneto are rotating even with notes. With a little bit of luck you'll be really close to the ballpark. I'll go through the process several times triple checking everything and even then not be satisfied I've done it right though my measurements come out ok.

Once satisfied with your results, you can move on to timing the magneto to 38* BTDC. This was it's typical fiddly job. Set the degree a hair before 38*, move the points until they are about to open, backup a touch, tighten the nut on the timing gear and check to see where its opening. Keep your special timing gear puller nearby as most of the time its not spot on the first few tries. 

Time to take a break and call it a night.  We'll double check everything in the AM...